1. Field of the Invention
The invention relates to a keypad scanning circuitry used in an electronic device, particularly a keypad scanning circuitry with an ESD protection function.
2. Description of the Prior Art
In recent years, kinds of portable consumer electronic products have been improved. Various fancy features and functions are added. Slim, compact and lightweight designs have become the mainstream of the market and even the norms of the industry. With regard to mobile communication devices such as cellular phones, variety of rich tone rings and color screen displays have long ago considered by consumers as the very basic of features. Keypads designed with ergonomics in mind are no longer a rarity and have often been a much advertised feature for many manufacturers. Foldable cell phones do not require the use of a set of lock keys to protect against unwanted depression of keys, while the flip lid, the upper part of the cell phone, can be used for accommodating such added features as digital cameras, timepiece, in-coming call number show-ups, and various message displays etc., As a result, flip cell phones have always enjoyed better popularity among consumers than non-flip cell phones.
To allow for easy operation of their cell phones, cell phone manufacturers often have some of their cell phones' operation keys built on the sides of the cell phone case, making it possible for the user to operate single-handedly such functions as dialing, recording, and volume controlling etc., However, as cell phones are being made smaller and smaller, the possibility that they get such severe electrostatic shocks as to damage their internal IC circuitry or the ITO cables inside the LCD display module is also increasing. As a result, the higher the resolution of a cell phone's LCD display is, the smaller the side keys become, and the more difficult the task of designing ESD protection for the phone will be.
When a naked side key of a cell phone is in direct contact with foreign objects in its surrounding, friction can cause electrostatic charges to be generated and accumulated, and later discharged through the side key into the cell phone's internal ICs, causing irreparable damages.
Another peril comes from the cell phone user. In a very dry environment, the very electrostatic a person generates can easily reaches 1 KV or 2 KV, which, if discharged through the side keys, is sufficient to cause havoc to the cell phone's internal ICs.
As a result, most manufacturers have to design ESD protection, in addition to magnetic disruption protection, into their cell phones. Some, for instance, provide silver foil shields for the internal parts of their cell phones. Others try to come up with new circuitry designs for their keypad scanning devices to combat the problem.
With regard to the latter way of solution, prior art keypad scanning devices have their IC matrix arranged on two sides: the input side and the output side—with row conductive lines on one side and column conductive lines on the other, and intersections of row lines and column lines set for detecting key depression. Thus, when, for example, the area where row line I (say, on the output side) intersects column line J (say, on the input side) is pressed, the two lines (the output line and the input line) will reach the same potential simultaneously, which the IC interprets as the corresponding key being pressed, and hence, immediately send out a signal. To provide for ESD protection, most manufacturers arrange their PC board layout in such ways that the intersecting areas 10, 20 of column and row lines shape like that of a doughnut. The area where resistance is low (the output side of IC) is arranged in the center; the area where resistance is high (the input side of IC) is arranged in the outer periphery, the consideration being that resistance on the output side is usually smaller than that on the input side, and ESD generally strikes through paths where resistance is smaller.
Although resistance on the input side is usually greater, which offers better protection against ESD, keys on the side of a slim design cell phone is often still susceptible to the strikes of ESD because the input line beneath these keys cannot accommodate ample contact area, which surrounds the output line's contact and protects it against ESD strikes. Thus protection is thus inadequate and is easy for ESD to strike through these side keys and damage the keypad encoder's IC. For this reason, some extra ESD protection is needed to ensure that the keypad encoder IC won't be damaged before input signals are transmitted into it.
The present invention introduces a new perspective to the current ESD protection thinking. It is not necessary for the internal parts of the cell phone to be equipped with ESD protection shields, just a little creative thinking and the problem can be substantially alleviated.